Networks of microlenses are currently used in diverse applications and, specifically, in image reproducers or sensors. In the former area of technology, video image projectors operate by projecting light through a 2-dimensional matrix of liquid crystal cells that display the image to be projected. These image projectors suffer from a low light yield due to the various light absorptions by polarizers located on either side of the matrix, by the opaque margins of the cells of the matrix, by the projection screen, etc. In this respect, it would be possible to improve the current light yield (on the order of 1%) by a factor of 2 or 3 by focusing the light from a source onto the useful areas of the liquid crystal cells, which are selectively switched between an opaque state and a transparent state. Absorptions by the opaque margins of these cells would thereby be prevented. This problem of absorption is encountered in other apparatuses such as the back-lit liquid crystal screens used in computers for displaying information.
Considering the dimensions of the liquid crystal cell matrixes used and the high definition currently envisioned for video images intended for the general public, we must then have 2-dimensional networks of microlenses distributed with a gauge on the order of 100 .mu.m, for example, the network having a number of lenses ranging from several hundred thousand to several million.
U.S. Pat. No. 4,572,611 makes such a mask using a composition of light-sensitive glass selectively ceramicizable by exposure to short-wavelength radiation, through a mask. The pads exposed contract and the stresses then applied to the unexposed adjacent pads have the effect of causing their surface to be bombarded, in such a way as to form microlenses.
In the application to improve the light yield of liquid crystal cell matrixes for a video image projector, this process is however not perfectly satisfactory, specifically because the coefficient of thermal dilation of the light-sensitive glass used (89.10.sup.-7 C.sup.-1, approximately) for making the network is very different from the one (46.10.sup.-7 C.sup.-1, approximately) of the glass that is today generally used for making the chamber of the liquid crystal cell matrix. We understand that a differential dilation of those glasses is able to disturb the rigorous alignment that must be maintained between each microlens and the cell of the matrix on which it must remain centered.
We are also familiar with a process for making networks of microlenses by ion exchange in a glass substrate. Unfortunately, the glass of the substrate has a coefficient of thermal dilation of the same order as that of the light-sensitive glass indicated above, resulting in the same problems of differential thermal dilation.
We also know how to make networks of microlenses by mechanical pressing of a plate of an optical material against a mold that precisely reproduces, in impression form, the surface of the network of microlenses. These processes have the disadvantage of requiring the construction of molds that are very delicate, and therefore expensive, to manufacture, with reduced longevity due to the fact that it is not possible to reuse such a mold once minimal alterations in its surface make it impossible to produce networks of lenses with an optical finish surface.
To make up for this last disadvantage, we are also familiar, from English patent No. 2,264,890, with a process for manufacturing networks of small-dimension lenses (1 to 2 mm in diameter) according to which a sheet of optical material (e.g. a polycarbonate, made plastic by heating) is pressed against a stainless steel plate pierced with a network of circular openings distributed and conformed like the contours of the lenses of the network to be produced. Under the effect of the pressure exerted on the sheet of optical material, it protrudes into the openings, assuming the bulge of a convex lens. Since the openings traverse the steel plate, with a thickness greater than that of the lenses to be formed, the bulging surfaces of the lenses do not come into contact with the steel plate and so cannot be altered by mechanical contact. Conversely, the face of the sheet of optical material that is opposite the one that bears the lenses is in physical contact, under pressure, with a sheet of glass or stainless steel that can deteriorate over time and print a replica of its surface defects onto the opposite face of the sheet of optical material. This is a disadvantage, specifically if we envision integrating the network of lenses into a glass plate comprising one of the walls of the chamber of a matrix of liquid crystal cells, which wall must have an optical finish so as not to disturb the functioning of the cells.
Therefore, the purpose of this invention is to make a network of microlenses having an optical finish on both of its faces.
The purpose of this invention is also to make such a network, designed to be combined with or incorporated into a matrix of liquid crystal cells for a video image projector or back-lit information display screen.